Mountaineering:  Freedom of the Hills is a book too few people who venture into the out-of-doors read.  
In my 5th edition, there is a section in the chapter on Wilderness Travel/Walking called "Etiquette."  Number six in a list of seven rules states:
"Step aside on the way downhill to let a party of uphill hikers continue forward without breaking their pace."
I've always followed that.  In practice, more often than not--as Harvey says--an uphill-going person is happy for an excuse to take a breather, and will step aside first.  The point is, make the gesture and then let the uphill person decide.